Demonstrate the differences between elements, mixtures and compounds and show how chemical changes are often accompanied by a large change in energy

In this demonstration a mixture of zinc and sulfur produces an unusual chemical reaction when heated. A brilliant flash of light, followed by hot sparks, a hissing sound and a mushroom-shaped cloud of white smoke are generated.

It can be used to demonstrate the following:

  • the properties of the products of a chemical reaction are quite different from the reactants
  • the products are difficult to separate to form the reactants, unlike mixtures
  • there is often a large change in energy involved in the formation of the product

The reaction between iron and sulfur is often used to demonstrate the same points and is suitable for a class practical. It could be used to reinforce the ideas following this demonstration. 

This reaction offers the same teaching opportunities as the Fe/S reaction but uses different techniques for identifying the nature of the reactants and products. Electrical conductivity can be used to show the differences between the elements, rather than magnetism, and a mixture of the reactants can be separated by reaction with water or dilute acid (if zinc dust is used). This provides a more detailed analysis of the chemistry involved in the reaction.

Lesson organisation

This experiment works well as a class demonstration. The demonstration takes about 5 minutes.

Apparatus and chemicals

  • Eye protection
  • Access to a fume cupboard
  • Test tube Pyrex (or boiling tube)
  • Metal test tube holder
  • Bunsen burner
  • Weighing boat
  • Spatula (2)
  • Top pan balance (1 dp)
  • 0.1 g Zinc powder (Highly flammable, Refer to SSERC or CLEAPSS Hazcard107 (see Technical note 1)
  • 0.1 g Sulfur powder (Irritant – skin, Refer to SSERC or CLEAPSS Hazcard96A (see Technical note 2)
  • 10 g Mineral wool

Technical notes

  1. Zinc powder or dust is very reactive and highly flammable – it must be stored and disposed of safely. It may be supplied in different states of fineness, and it may have become oxidised and be mainly zinc oxide. For that reason the reactivity seen from any given sample can be very different.
  2. Sulfur may be supplied as crushed roll sulfur, flowers of sulfur, precipitated sulfur or resublimed sulfur. All are suitable, but resublimed sulfur seems to react more vigorously.

Procedure

HEALTH & SAFETY: Wear eye protection, do reaction in a fume cupboard

The demonstration

a. Measure out 0.1 g of zinc powder into a weighing boat.

b. Measure out 0.1 g sulfur powder into the weighing boat.

c. Mix the two powders to form a uniform mix.

d. Put the powder into a Pyrex test tube.

e. Fit a mineral wool plug to the top of the test tube.

f. Light the Bunsen and adjust to a blue working flame.

g. Holding the tube with the test tube holders, heat the mixed powders and direct the mouth of the tube towards the inner corner of the fume cupboard, until the reaction occurs.

Disposal

Put the remnants from the reaction into a beaker of 500 cm3 of dilute hydrochloric acid (0.1 molar) and leave for an hour or so (stirring from time to time). This will dissolve any remaining metal (and the oxide). Then neutralise the acid and wash to waste with plenty of running water.

Teaching notes

The products of this reaction bear little resemblance to the starting elements, pale blue zinc and bright yellow sulfur. Students can see that a new compound has been formed. The pale yellow residue is a mixture of zinc oxide and zinc sulfide. Reducing this residue (by electrical or chemical means) back to zinc demonstrates the chemical differences between mixtures and compounds. A drop of hydrochloric acid on the residue can be a fitting conclusion to the reaction, if done in a fume cupboard. The reaction produces the pungent, foul-smelling gas hydrogen sulfide which students often associate with stink bombs – once smelled never forgotten. 

Zinc is an essential element – the mineral is required for skin and bone growth, and our bodies use zinc to process food and nutrients. Zinc ions are vital components in several different enzymes found in the body. A pale yellow, odourless, brittle solid, sulfur is essential to life too, occurring in the amino acids cysteine and methionine and therefore in many proteins. It is a minor constituent of fats, body fluids, and skeletal minerals.

The chemical reactions which are occurring in the reaction are:

Zn(s) + S(s) → ZnS(s) ΔHf = –206.0 kJ mol-1

Zn(s) + ½O2(g) → ZnO(s) ΔHf = –348.3 kJ mol-1

S(s) + O2(g) → SO2(g) ΔHf = –297.0 kJ mol-1

The overall reaction needs heat to get started but the heat it produces is enough to sustain the reaction thereafter.

Zinc can be obtained by electrolysis of zinc sulfate or by smelting in a process similar to the production of iron from the blast furnace. First the zinc ore is roasted in air, converting it to zinc oxide:

2ZnS(s) + 3O2(g) → 2ZnO(s) + 2SO2(g)

Coke and the roasted ore are fed into the top of the furnace with air blasted in at the bottom. The most important reaction taking place is:

ZnO(s) + CO(g) → Zn(g) + CO2(g) 

More resources

Unlike the production of iron (mp = 1535°C; bp = 2750°C), zinc (mp = 420°C; bp = 907°C) is produced as a vapour. Cooling the zinc vapour to produce a liquid results in the re-oxidation of the metal. This problem was solved in the 1950s by Imperial Smelting of Bristol. The zinc vapour is sprayed with molten lead. This chills and dissolves the zinc so rapidly that re-oxidation is minimal. Molten lead and zinc are only partially miscible in each other and so, by cooling the solution, zinc separates as a liquid of nearly 99% purity. Vacuum distillation can further refine the liquid to 99.99% purity. This method has the advantage that the charge composition is not critical and mixed Zn/Pb sulfide ores (often found together) will produce both metals simultaneously, the lead being tapped from the bottom of the furnace.

When using this demonstration, it is worth mentioning that zinc is applied in thin layers to iron and steel products to stop them rusting. This process is called galvanising. More than half of the zinc consumed each year is used for galvanising. About 7.7 kg of zinc is used to protect the average car from rust.

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